Centralized supervision and control of today's complex electric power systems has created the need for devices capable of converting the significant AC power system quantities into accurate proportional signals that can be easily transmitted, recorded and fed into data acquisition systems. These signals can also be used for process control where, for example, the power consumed is directly proportional to motor torque. For measurement of the main quantity, AC power, various watt transducers have been developed. A watt transducer converts AC watts into a DC current which is directly proportional to AC watts. It is a common misconception to assume that watt and var transducers sense power. This is not true. These devices sense voltage and current and subsequently compute power from those quantities. There are various methods of effecting that computation; the time division multiplication, perhaps more accurately termed pulse width-pulse height multiplication, method is perhaps the most accurate contemporary analog multiplication technique. The theory of this power computation method is based upon the principle that one input signal controls the width or duty cycle of a pulse train while a second input signal controls the amplitude of that pulse train. After the width and amplitude of the pulse train have been controlled (modulated) by the two input signals, the pulse train then passes through a low pass filter which retains only the DC component of the pulse train. Since the area of a rectangle is length.times.width, the resulting DC output voltage is therefore proportional to the product of the two input signals.
Of paramount importance in devices of this type is stability and accuracy of the performance of the device over changes in temperature and passages of time. The height of the triangular wave at the output of the triangular wave generator directly effects the stability of the overall performance of the device. The amplitude of this triangular wave must be extremely stable or the accuracy of the transducer will drift with temperature or over the passage of time. The amplitude of the triangular wave is, in turn, directly proportional to the amplitude of the squarewave input signal to the integrator within the triangular wave generator. Such squarewave inputs are traditionally produced by an output from a comparator in a feedback loop. The output of such a comparator essentially switches between its two supply voltages, which arrangement suffers from two problems: (1) The output circuit of the comparator often drifts with temperature changes; and, (2) Since the comparator switches between two supply voltages, it is necessary that the supply voltages themselves be very stable with changes in temperature and over passages of time.
It is, therefore, an object of this invention to provide a stable and accurate output over changes in temperature and passage of time by ensuring a uniform squarewave input to the integrator within the triangular wave generator which produces a triangular wave output such that the amplitude of that triangular wave output is extremely stable over changes in temperature and passage of time.
Another problem encountered with watt transducers of the pulse width-pulse height modulation multiplication type is the adjustment of the sensitivity of the transducer for various configurations used in power monitoring, such as 1, 2, and 3 element power monitoring. If one designs a one-element, single phase transducer to produce an output of one milliamp at 500 watts (a standard output recognized in industry), then for a three-phase, four-wire, three-element transducer, one must adjust the sensitivity of each of the three elements, since they are summed, to one-third of their one-element value in order to produce the same output current when the transducer sees a total of 1500 watts (500 watts per element). If this were not accomplished, the transducer would produce three times the output current for a three-element application as it would for a one-element application. The heretofore unsolved problem has been determining exactly where to adjust the sensitivity of each element in order to produce one-third of the output when three times the input is present.
It is, therefore, a further object of this invention to provide a transducer which is very simply adjustable to various multi-element applications without sacrificing stability or accuracy and without expensive redesigning of circuitry or reworking in a manufacturing facility.
Still another problem with transducers of this type is that they are difficult to calibrate and maintain after their installation. Equipment which is capable of controlling all three factors involved in computation of power (voltage, current and power factor) is expensive to produce and difficult to operate except within a highly controlled environment, such as a calibration laboratory. Further, transducers of this type presently available are complexly configured and generally must be returned to the manufacturer for repair.
It is, therefore, a still further object of this invention to provide a transducer capable of measuring alternating as well as non-alternating electrical signals and computing power therefrom, thereby allowing calibration of that transducer with non-alternating electrical signals which involves only two parts in power computation (voltage and current). Simpler, cheaper and more accurate calibration is thereby attainable.
It is yet another object of this invention to produce a transducer constructed in such a manner that such a device installed in the field can be easily repaired by insertion of pre-calibrated circuitboards on site with no sacrifice suffered in stability, accuracy of the device, or equipment downtime.
Further objects and features of the present invention will be apparent from the following specification and claims when considered in connection with the accompanying drawings illustrating the preferred embodiment of the invention.